Hi Christian,
What I normally use is the following :
1) Use GetEntry(ient) to get a certain entry in memory.
2) Then do something like Event* evtold=new Event(*event)
to store the current event into the evtold structure.
3) Then do again GetEntry(ient2) to obtain a different
event in memory.
4) Now you have 2 structures filled, namely "event"
and "evtold" and you can have fun with your mixing :).
Cheers,
Nick.
--
Dr. Nick van Eijndhoven mailto:nick@phys.uu.nl http://www.phys.uu.nl/~nick
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Christian Holm Christensen wrote:
>
> Hi ROOT'ers,
>
> I have a question on how to do mixed event analysis using TChains.
>
> The point of it all is, that given that you're in some event, you'd
> like to read another event too, be it a previous or a later event. So
> in terms of (meta-) code, it be something like:
>
> DO WHILE there are more events to read
>
> read next event into memory
>
> read a previous or later event into memory too
>
> process the two events
>
> END
>
> or in a more ROOT like way:
>
> TChain* chain = ...
> chain->Add(...
> chain->SetBranchAddress("Event", &event);
> ...
>
> while (chain->GetEntry(i++)) {
> ...
> chain->SetBranchAddress("Event", &other);
> chain->GetEntry(j);
>
> mix_events->(event, other);
>
> chain->SetBranchAddress("Event", &event);
> }
>
>
> I think the problem is, that TChains are a bit particular on how you
> access the data in them - they want you to read the files that make up
> the chain sequentially - that is, you can not cross file boundaries.
>
> I made an (simple) example, which is available at
>
> http://cholm.home.cern.ch/cholm/root/mixevent.tar.gz
>
> Take a look at that code. Try it as follows:
>
> > tar -xzvf mixevent.tar.gz
> > cd mixevent
> > make check
>
> As it is now, it will fail, as it's reading two events ahead, hence
> crossing file boundaries. Now, try running it skipping only one event
> ahead
>
> > ./Read 1
>
> That should work. Now, try running it again, reading some events
> backward:
>
> > ./Read <any negative number>
>
> No matter what the absolute value of <any negative number> is, as long
> as it's negative, it will always fail when you're reading back over a
> file boundary.
>
> I can think of 3 ways of working around this:
>
> 1) Make the user set a flag that caches the current event, and they
> can then call it back from memory storage later on. This obviously
> doesn't work if you want to read events ahead of the current one.
>
> 2) Always keep say 5 events in memory, and let the `current' point to
> the middle one. Hence the user can use up to 2 events
> forward/backward - this behaviour could be dependable one a runtime
> flag of course.
>
> 3) Allow unlimited access to previous or next events, as long as they
> are within the same file as the current event. I believe there's a
> method to get the index in the current tree rather than in the
> chain (I think it's used when you do TChain::MakeClass).
>
> I personally prefer option 3.
>
> Now my question: What am I doing wrong here? Have I missed something
> in the documentation? Is there anyway to do this kind of thing more
> naturally, than the 3 work-a-rounds? If not, could there be?
>
> Thank you.
>
> Yours,
>
> ___ | Christian Holm Christensen
> |_| | -------------------------------------------------------------
> | | Address: Sankt Hansgade 23, 1. th. Phone: (+45) 35 35 96 91
> _| DK-2200 Copenhagen N Cell: (+45) 24 61 85 91
> _| Denmark Office: (+45) 353 25 305
> ____| Email: cholm@nbi.dk Web: www.nbi.dk/~cholm
> | |
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